Cathode ray tube with improved electron gun structure

ABSTRACT

A cathode ray tube is provided with an electron gun structure including a cathode, a small apertured grid, and a small apertured anode on the remote side of the grid from the cathode. In addition, a further electrode is located between the aforementioned grid and the anode. A grid electrode, disposed adjacent and preferably mounted on the grid, is provided with an aperture coaxial with the grid aperture, but this aperture has a diameter effectively several times the diameter of the grid aperture. An anode electrode is also located between the grid and the aforementioned anode and is suitably connected to approximately the same voltage as the anode. The anode electrode is similarly provided with an aperture coaxial with the other apertures, but this aperture is also several times greater in diameter than the small aperture in the grid. The anode electrode acts to strengthen the electric field between such anode electrode and the grid whereby to provide a more concentrated electron beam. The region between the anode electrode and the anode provides a divergent lensing action for producing a small electron beam spot size at the screen end of the cathode ray tube.

United States Patent [72] Inventor ConradJ.Odenthal Beaverton, Oreg. [21] AppLNo. 720,035 [22] Filed Apr. 10, I968 [45] Patented Mar. 2, 1971 [73] Assignee Tektronix, Inc.

Beaverton,0reg.

[54] CATHODE RAY TUBE WITH IMPROVED ELECTRON GUN STRUCTURE 13 Claims, 5 Drawing Figs. [52] U.S.Cl 315/15, 313/82,3l5/16 [51] Int.Cl I101j29/46 [50] FieldofSearch ..315/14,15, 16;313/82 [56] References Cited UNITED STATES PATENTS 2,225,917 12/1940 Mahl 313/82 2,922,072 l/1960 Collinsetal. 315/16 2,971,108 2/1961 Dickinsonetal... 3l5/16X 3,004,186 10/1961 Gray 315/15 3,047,759 7/1962 McNaney 313/82 500 TO 800V.

O TO-IOOV Pn'mary Exa'miner--Rodney D. Bennett, Jr. Assistant Examiner-Malcolm F. l-lubler Attorney- Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: A cathode ray tube is. provided with an electron gun structure including a cathode, a small apertured grid, and a small apertured anode on the remote side of the grid from the cathode. In addition, a further electrode is located between the aforementioned grid and the anode. A grid electrode, disposed adjacent and preferably mounted on'the grid, is provided with an aperture coaxial with the grid aperture, but this aperture has a diameter effectively several times the diameter of the grid aperture. An anode electrode is also located between the grid and the aforementioned anode and is suitably connected to approximately the same voltage as the anode. The anode electrode is similarly provided with an aperture coaxial with the other apertures, but this aperture is also several times greater in diameter than the small aperture in the grid. The anode electrode acts to strengthen the electric field between such anode electrode and the grid whereby to provide a more concentrated electron beam. The region between the anode electrode and the anode provides a divergent lensing action for producing a small electron beam spot size at the screen end of the cathode ray tube.

PATENTED MAR 21971 Fr -d loan:

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INVENTOR L A H T N E D O J D A R N O C N VN NN BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS i CATHODE RAY TUBE WITH IMPROVED ELECTRON GUN STRUCTURE BACKGROUND OF THE INVENTION In a popular form of electron gun for cathode ray tubes, an apertured grid electrode cooperates with an anode to produce a crossover of the tubes electron beam near the aperture in the grid. Such crossover is then imaged on the cathode ray tube screen by the lensing action of remaining gun electrodes. It is, of course, desired that the electron gunoperate with optimum efficiency whereby as large a proportion as possible of the electrons emitted at the cathode reach the cathode ray tube screen. Moreover, it is frequently desired that the image of the electron beam crossover at the cathode ray tube screen, that is, the spot size, be as small as possible while at the same time representing a large beam current for producing a bright spot.

Frequently a converging electron lens is located immediately on the screen side of the cathode ray tube guns grid. This lens tends to concentrate the beam somewhat but it does not ordinarily have the property of producing a virtual image smaller than the crossover, but rather the virtual image is sometimes larger than such crossover. Because of the use of beam limiting apertures in the gun, it should be noted that requirements of high beam current and small spot size are usually contradictory. Thus, a converging lens may produce higher beam current through the stopping aperture, but often has an undesired effect on spot size, while a diverging lens tends to decrease spot size but at the same time widens the beam which has more of its current stopped out by the limiting aperture.

SUMMARY OF THE INVENTION In accordance with the present invention, a cathode ray tube includes an electron gun structure having the usual cathode, grid, and anode elements. The grid and anode are provided with small apertures through which the electron beam passes. A beam crossover, or circle of least confusion, is produced near the grid aperture, while the anode accelerates an electron beam on toward the cathode ray tubes screen where such crossover is focused. The present invention further includes a grid electrode means and an anode electrode means located between the aforementioned grid and anode. The grid electrode means is located adjacent and is preferably attached to the grid electrode, and such grid electrode means is provided with an aperture coaxial with the grid aperture but having a diameter several times the diameter of the small aperture in the grid. The anode electrode means, which is located closer to the anode than is the grid electrode means, is also provided with an aperture coaxial with the other apertures, but having a diameter several times that of the grid aperture. The anode electrode means aperture is also preferably larger than the grid electrode means aperture. The anode electrode means is connected to a potential nearer the potential of the anode electrode than the potential of the grid electrode, and acts to strengthen the electric field between such anode electrode means and the grid electrode. As a result, the strong field between the grid and the anode electrode means rapidly concentrates the electron beam while resulting in substantially no magnification thereof. Then, a divergent lens action between the anode electrode means and the anode produces a smaller virtual image of the beam crossover for focusing upon the cathode ray tube screen. It has been found that this structure produces an appreciable increase in beam current with a reduction in spot size, while cathode loading remains substantially the same.

It is accordingly an object of the present invention to provide a cathode ray tube with an electron gun exhibiting improved performance, i.e. providing a smaller spot size with increased beam current and without substantial increase in cathode loading.

It is a further object of the present invention to provide a cathode ray tube with an improved electron gun producing a stronger, more concentrated electron beam, wherein the effects of space-charge spreading of the electron beam are minimized.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements:

DRAWINGS FIG. 1 is a cross-sectional view of a portion of a prior art electron gun;

FIG. 2 is a side elevational representation of internal structure of a cathode ray tube according to the present invention;

FIG. 3 is a cross-sectional view of a portion of an electron gun according to the present invention;

FIG. 4 is a plan view, partially broken away, of the gun portion illustrated in FIG. 3; and

FIG. 5 is a cross-sectional view of an electron gun structure according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION Referring to FIG. 2, the internal construction of a cathode ray tube according to the present invention includes a cathode l0 surrounded by grid 12 in the form of a grid cup having a small aperture 14 through which electron beam 16 is emitted. The electron beam travels from aperture 14 through central aperture 18 at one end of anode barrel or first anode 20. The opposite end of first anode 20 is open and coaxial with focus ring 22 which in turn faces the open end of second anode 24. Second anode 24 is provided with an aperture stop 26 at the end thereof facing the screen 31 at the screen end of the cathode ray tube. After passing through second anode 24, beam 16 passes between a first pair of deflection plates 28 and a second set of orthogonally oriented deflection plates 30, the function of which is to move the electron beam horizontally and vertically. Typical voltages for the various electrodes are indicated on the drawing. Thus, the first anode 20 is at a high positive voltage with respect to the cathode and grid, and therefore strongly attracts electron beam 16 which passes through aperture 18. Grid 12, which may be biased at a voltage somewhat lower than cathode 10, not only regulates the amount of current in beam 16, but also provides a lensing action at aperture 14 for directly determining the size and position of an electron beam crossover. In order to produce a small beam spot at the screen of the cathode ray tube, this crossover is desirably as small as possible, i.e. the electrons are ideally converged to a sharply defined point. However, since the electrons emitted from cathode 10 are nonparallel, and come off the cathode in random directions with varying velocities, a sharp point is not established. However, the electrons do converge into a smaller area or circle of least confusion normally called the crossover. This crossover is imaged on the screen of the cathode ray tube by the electron optics of the system.

FIG. 1 illustrates a prior art emitting portion of a commonly employed electron gun structure, wherein like elements are referred to by the same reference numerals used in FIG. 2. Here the electron beam crossover is indicated at 32. Between the grid 12, which may be at 0 volts or at a somewhat negative voltage, and anode 20, there is established an electric field 34 indicated by lines substantially parallel to grid 12 and anode 20 in FIG. 1. This field may be described as a converging field inasmuch as the electrons of beam 16 are accelerated thereby, and tend more in an axial direction .36 as they pass through this field. The convergent field therefore has the property of concentrating the electron beam after crossover 32 so that at least a substantial portion thereof may pass through apertures 18 and26. The converging field of this type ordinarily has little magnifying or demagnifying effect so far as the virtual image of the crossover is concerned, but does place the virtual image further behind the cathodesurface, and, to the focusing sectiori22 of the electron gun, theelectronsappear to be coming from this virtual image. Because of the ratio of screen distance to focus section 22 over the distance of focus 22 to the virtual image position, the virtual image is projected onto the screen 31 magnified by the ratio of these two distances. And since in the usual CRT the distance from the focus section 22 to the screen 31 is several times larger than the distance from focus22to virtual image, the spot onthe screen is usually several times larger than the crossover itself.

In the region of the crossover, an appreciable space-charge effect takes place, caused by the close bunching of the electrons in this region. This space-charge produces spreading of the electron beam .16 in addition to the optical spreading as may take place in the electron beam beyond the crossover or in the direction of arrow 36, thereby tending to reduce beam current passing through the aperture stop 26.

Enlargement of the beam by space-charge effects or the like can be reduced somewhat by bringing the anode closerjto grid'12 whereby the field gradient of field 34 is increased. However, the grid to anode spacing in a given tube is ordinarily determined by the cathode loading of and cutoff voltage permissible or desired. Just bringing the anode closer to the grid increases the cutoff voltage of the tube and increases the cathode loading. Thus, for a given tube, the position of anode 20 relative to grid 12 may be relatively fixed, depending on the cathode loading or anode voltage required.

According to the present invention, electron beam current is increased, and the spot size produced by the beam is reduced without changing the cathode loading. Thus,'for a given tube, and a given amount of cathode loading, the gun performance is enhanced. The emitting end of the cathode ray tube electron gun in accordance with the present invention is illustrated inFIG. 3, and also the additional elements thereof j are included in the tube structure of FIG. 2. In addition to the conventional cathode 10, grid 12, and anode 20, the structure further includes a grid electrode means38 disposed adjacent grid 12 between grid 12 and anode 20. The grid electrode means 38 is maintained at a potential nearer to the potential of the grid 12 than to the potential of anode 20. The potential of the grid electrode means is at least between the potential of the grid 12 and the potential which would correspond to the position of said grid electrode means in a uniform field between the first anode and the grid, assuming such uniform field existed. The grid electrode electrode'means 38 is suitably a thin conducting metal wafer or disc of conducting'rnaterial and is providedwith a central aperture 40 coaxially aligned with aperture 14 and with electron beam 16. However, the diameter of aperture 40 is several times the small diameter of aperture 14. The thickness of the wafer comprising the grid electrode means is suitably comparable with the diameter of aperture 14 in the grid electrode.

In the illustrated embodiment, and preferably in actual practice, grid electrode means 38 is mounted immediately upon grid 12 whereby the grid-electrode means 38 forms an annular shoulder portion surrounding aperture 14 but spaced radially therefrom. Thus, in this case, grid electrode means 38 resides at the same potential as grid 12, i.e. as selected by potentiometer 44 connected between ground and a minus 100 volts. Aperture 40 is suitably cylindrical and extends axially in the direction of arrow 42 to the extent of the width of the wafer or disc.

Additionally, ana'node electrode means 46 is supported between grid l2 and anode 20, and at least a portion thereof is desirably closer to anode 20 than is any portion of grid electrode means 38. Anode electrode means46 suitably comprises a thin conducting metal disc or wafer disposed between grid electrode means 38 and anode 20. 'Anode electrode means 46 is provided with a central aperture 48 coaxially aligned with the apertures of the other electrodes and with the electron beam 16and has an inside diameter larger than the inside diameter of aperture 14, or of aperture 18 which is comparable in size with aperture 14. The inside diameter of aperture 48 is also preferably greater than the inside diameter of aperture 40 of grid electrode means 38. Aperture 48 is also suitably cylindrical in the direction of the electron beam and arrow 42 through the width of anode electrode means 46, wherein such width is comparable to the diameter of aperture 14. Although cylindrical apertures in electrode means 38 and 46 achieve the best results, it is possible that these apertures be other'than uniformly cylindrical, so long as the effective diameters of these'apertures are appreciably larger than the diameter of apertures l4 and l8.

. Anode electrode means 46 is connected to a potential nearer to the potential of the first anode 20 than to the potential of the grid'12. The potential of anode electrode means is at least between the potential of the anode and the potential which would correspond to the position of the anode electrode means in a uniform field between the anode and the grid, assuming such uniform field existed. Under these circumstances, the anode electrode means 46'strengthens the electric field between itself and grid 12, compressing the equipotentials therebetween, as can be seen in FIG. 3. The anode electrode means 46 is most advantageously connected to anode 20 so as to reside at the same potential therewith.

Since the field is strengthened in region 50 between anode electrode means 46 and grid 12, or has a higher gradient therebetween, the electron beam 16"will be more greatly accelerated in region 50 than in the corresponding portion of field 34 in FIG. 1. Therefore, electron beam 16' will be concentrated more densely, resulting in a narrower beam by the time it reaches field region 52. This'added concentration of the beam results from two factors. First, the greater field gradient produces a more convergent lensing action'resulting in less widening of the beam that would otherwise take place after crossover 32. Second, the added acceleration of the electrons of electron beam 16 in region 50 aids in overcoming space-charge spreading of the beam as would otherwise occur near and subsequent to crossover 32. Therefore, by the time the beam reaches region 52 between anode electrode means 46 and anode 20, the beam is more concentrated whereby substantially more beam current may later pass through aperture 26 and reach the screen end ofthe cathode ray tube. As

the beam passes' through region 52, a divergence takes place means 46 andanode 20. The diverging property of the field produces a smaller virtual image of the crossover (or of the crossover image already produced bythe field in region'50) which is then focused at the screen end of the tube. This smaller virtual image is produced in the same manner that a diverging optical lens produces a smaller virtual image of an object viewed therethrough. Since the beam is well concentrated before entering region 52, this divergence together with the action of aperture stop 26 does not detract materially from the beam current reaching the screen end of the tube or nearly as much as a divergence taking place after the beam has spread further.

The'field in region 50 is considered a converging field although the field equipotentials are substantially parallel to one another and to grid l2 and anode 20 so far as the smaller diameter electron beam is concerned at that point. This field narrows the electron beam in the sense that the electron beam, because of acceleration, does not spread as much as it otherwise would. The converging field formed of substantially paralleland planar equipotentials does not produce magnification, i.e. a larger virtual image of the crossover. Rather, the virtual image produced by the field in region 50 is approximately the same size as the crossover, but the field in region 52 then produces demagnification thereof.

It should be noted that the increased beam current and decreased spot size provided by the present structure is accomplished without increase in cathode loading. It is postulated that grid electrode means 38, residing at a low potential, i.e. the potential of the grid, holds the compressed electric field in region 50 from being forced closer to aperture 14. Thus, it will be observer observed that equipotential 54 in FIG. 3 is no closer to aperture 14 than is corresponding equipotential 54 associated with the FIG. 1 structure. Without grid electrode means 38, the field would be compressed closer towards the cathode, resulting in increased cutoff voltage, undesirably high cathode loading, and undesirable electron optical effects. So far as the cathode is concerned, the field draw-.

ing the electrons therefrom appears the same in either the FIG. 3 or the FIG. 1 structures. Therefore, the increased beam current with decreased spot size can be achieved without excessive cathode loading or the like.

The following table illustrates improved advantages obtained in a typical gun structure constructed in accordance with the FIG. 3 embodiment, as compared with one constructed in accordance with the prior art FIG. 1 arrangement:

TABLE I Fig. 1 Fig. 3

Cut-ofi voltage, v 76 76 I at Bias, ma 2.3 2.3 Spot size at 0 bias, mil 100 60 It/spot size at "0 bias, a/mil 4. 0 10. 0 Percent gun efiiciency 16. 5 21. 7 11, at "0 bias, a 400 500 In this table, data was taken with the first anode 20 at a positive 3.5 kv with respect to the cathode. Cutoff voltage is taken as the value of grid voltage needed to visually extinguish the spot. Spot size in mils was measured by the shrinking raster method. I is the cathode current in milliamperes at zero grid bias. 1,, is the corresponding portion of the cathode current which passes through stop aperture 26 to screen 31. Gun efficiency is the current reaching the screen divided by current leaving the cathode times 100.

In a typical embodiment, the construction of FIG. 3 had the dimensions given in the following table:

TABLE II Spacing between cathode 10 and top of grid 12 inches 0. 007 Thickness of grid 12 do 0. 010-0. 003 Diameter of aperture 14 do 0. 022 Thickness of wafer 38 do 0. 025 Thickness of wafer 46 do 0. 025 Spacing between wafer 38 and wafer 4 inches- 0. 030 Spacing between wafer 46 and the end of anode .inches- 0. 125 Diameter of aperture 18 do 0. 040 Diameter of aperture 40 do 0. 200 Diameter of aperture 48 do 0. 250 Diameter of aperture stop 26. do 0. 040

The above dimensions are merely given by way of specific example of a typical construction. In general, the aperture 40 in grid electrode means 38 should be approximately 8 to 10 times larger than that of aperture 14. Aperture 48 should in turn be larger than aperture 40, for example, approximately 1.25 to 1.5 times the diameter of aperture 40. Anode electrode means 46 should also be as close as possible to the grid electrode means 38 without producing arcing therebetween at the voltages employed. The aperture 18 in the anode is standard and is small, e.g. approximately 2 or 3 times the diameter of the grid aperture 14.

The particular construction of FIG. 3 is subject to a number of variations. For example, referring to FIG. 5, illustrating an alternative embodiment, grid electrode means 38 has an outer diameter not much greater than its inner diameter wherein the inner diameter provides the aperture 40. Grid electrode means 38 is an annular ring mounted on grid 12. Then an anode electrode means 46' is disposed outside the outer diameter of grid electrode means 38. Anode electrode means 46 thus is disposed at least partially in overlapping relation to the grid electrode, but, as will be appreciated, the same will act to compress the electric field close to grid aperture 14 without increasing cathode loading. Also, a divergent field will be provided towards aperture 18 in anode 20. Other variations of the structure according to the present invention will occur to those skilled in the art.

While I have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit. and scope of my invention.

I claim:

1. A cathode ray tube in which the improvement comprises an electron gun structure including:

a cathode for emitting an electron beam;

a grid electrode adjacent the cathode and having a small aperture through which the electron beam travels;

a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam;

means for supplying a relatively high voltage to the first anode electrode which is positive relative to the cathode;

said structure further including means to strengthen the electric field by compressing equipotentials between the anode electrode and the grid electrode;

and means for holding the strengthened electric field from the grid electrode toward the anode electrode for preventing excessive cathode loading; and

said means for holding the strengthened electric field from the grid electrode comprising grid electrode means proximate the side of said grid electrode where said electron beam exits, said grid electrode means. having an aperture through which said electron beam passes, which aperture is at least 10 times the diameter of said aperture in said grid electrode, said grid electrode means having a potential applied thereto no greater than a potential between the potential of said grid electrode and the potential which would correspond to the position of said grid electrode means in a uniform field between said grid electrode and said first anode electrode.

2. A cathode ray tube in which the improvement comprises an electron gun structure including:

a cathode for emitting an electron beam;

a grid electrode adjacent the cathode and having a small aperture through which a predetermined electron beam travels, said grid electrode cooperating to produce an electron beam crossover adapted for focusing upon the screen end of said cathode ray tube;

a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam, said anode having an aperture for at least substantially passing said electron beam;

means for supplying a relatively high voltage to the first anode electrode which is positive relative to the cathode;

grid electrode means positioned between said first anode electrode and said grid electrode in substantial contact with said grid electrode, said grid electrode means being provided with an aperture coaxial with the path of said electron beam, such aperture in the grid electrode means having an effective diameter at least approximately 8 to 10 times the diameter of the small aperture in said grid electrode, said grid electrode and said grid electrode meansboth being negative in potential relative to said cathode; and

an anode electrode means positioned between said first anode electrode and said gridelectrode, said anode electrode means being provided with an aperture coaxial with the path of the electron beam, such aperture in the anode electrode means having an effective diameter several times the diameter of the small aperture in said grid electrode, and means for connecting said anode electrode means to a potential nearer to the potential of the first anode electrode than to the potential of the grid electrode, said anode electrode means acting to strengthen the electric field between the anode electrode means and the grid electrode, while the predetermined electron beam remains attracted toward said first anode electrode. 3.The cathode ray tube according to claim 2 wherein said anode electrode means overlaps and is at least partially in the same plane with the said grid electrode means, being disposed at least partially in surrounding relation to said grid electrode means.

4. The cathode ray tube according to claim 2 wherein said anode electrode means is connected to substantially the same potential as said anode electrode.

5. The cathode ray tube according to claim 2 wherein said grid electrode means comprises an annular shoulder portion mounted on said grid electrode and extending axially of the electron beam toward said anode electrode in surrounding relation to the aperture in said grid electrode.

6. The cathode ray tube according to claim 2 wherein the aperture in said anode electrode means is larger in diameter than the aperture in said grid electrode means.

7. The cathode ray tube according to claim 2 wherein said grid electrode means and said anode electrode means each comprise a thin disc provided with the respective apertures therein, each such disc having a thickness on the order of magnitude of the diameter of the aperture in the grid electrode.

8. The cathode ray tube according to claim 2 wherein the potential of said anode electrode means is between the potential of said first anode electrode and the potential which would correspond to the position of said anode electrode means in a uniform field between the first anode electrode and the grid electrode.

9. The cathode ray tube according to claim 2 wherein said anode electrode means is closer to the first anode electrode than is said grid electrode means.

10. the cathode ray tube according to claim 9 wherein the anode electrode means is more closely spaced to said grid electrode means than to said first anode electrode 11. A cathode ray tube in which the improvement comprises an electron gun structure including:

a cathode for emitting an electron beam;

a grid electrode adjacent the cathode and having a small aperture through which a predetermined electron beam travels, said grid electrode cooperating to produce an electron beam crossover adapted for focusing upon the screen of said cathode ray tube;

a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam, said anode having an aperture for substantially passing said electron beam;

means for supplying a relatively high voltage to the first anode electrode which voltage is positive relative to the cathode;

annular, substantially disc-shaped grid electrode means positioned between said first anode electrode and said grid electrode an adjacent said grid electrode, said grid electrode means being provided with an aperture coaxial with the path of said electron beam, such aperture in the grid electrode means having a diameter several times the diameter of the small aperture in said grid electrode, and means connecting said grid electrode means to the same potential as the grid electrode;

and annular, disc-shaped anode electrode means positioned between said anode electrode and said grid electrode means in closer s aced relation to said grid electrode means than to sar first anode electrode, said anode electrode means being provided with an aperture coaxial with the path of the electron beam, and means for connecting said anode electrode means to substantially the same potential as the anode electrode, said anode electrode means acting to strengthen the electric field between the anode electrode means and the grid electrode, While the electron beam remains attracted toward said first anode electrode.

12. The cathode ray tube according to claim 11 wherein said grid electrode means comprises an annular shoulder mounted on said grid electrode.

13. The cathode ray tube according to claim 12 wherein said anode electrode means overlaps and is at least partially in the same plane with the grid electrode means, being disposed at least partially in surrounding relation to said grid electrode means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 557 991 Dated M h 2 1971 Inventor s) CONRAD J ODENTHAL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Abstract, line 4, "a further electrode is" should be -fur electrodes are-- Column 3, line 27, delete "of".

Column 5, line 11, delete "observer".

Column 7, line 46, "the" should be -The--.

Column 8, line 2 insert a period after "electrode";

2 line 18, delete one of the consecutive commas, line 20, "an" should be --and-- Signed and sealed this 30th day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patent FORM PC1-1050 (10-89) 

1. A cathode ray tube in which the improvement comprises an electron gun structure including: a cathode for emitting an electron beam; a grid electrode adjacent the cathode and having a small aperture through which the electron beam travels; a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam; means for supplying a relatively high voltage to the first anode electrode which is positive relative to the cathode; said structure further including means to strengthen the electric field by compressing equipotentials between the anode electrode and the grid electrode; and means for holding the strengthened electric field from the grid electrode toward the anode electrode for preventing excessive cathode loading; and said means for holding the strengthened electric field from the grid electrode comprising grid electrode means proximate the side of said grid electrode where said electron beam exits, said grid electrode means having an aperture through which said electron beam passes, which aperture is at least 10 times the diameter of said aperture in said grid electrode, said grid electrode means having a potential applied thereto no greater than a potential between the potential of said grid electrode and the potential which would correspond to the position of said grid electrode means in a uniform field between said grid electrode and said first anode electrode.
 2. A cathode ray tube in which the improvement comprises an electron gun struCture including: a cathode for emitting an electron beam; a grid electrode adjacent the cathode and having a small aperture through which a predetermined electron beam travels, said grid electrode cooperating to produce an electron beam crossover adapted for focusing upon the screen end of said cathode ray tube; a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam, said anode having an aperture for at least substantially passing said electron beam; means for supplying a relatively high voltage to the first anode electrode which is positive relative to the cathode; grid electrode means positioned between said first anode electrode and said grid electrode in substantial contact with said grid electrode, said grid electrode means being provided with an aperture coaxial with the path of said electron beam, such aperture in the grid electrode means having an effective diameter at least approximately 8 to 10 times the diameter of the small aperture in said grid electrode, said grid electrode and said grid electrode means both being negative in potential relative to said cathode; and an anode electrode means positioned between said first anode electrode and said grid electrode, said anode electrode means being provided with an aperture coaxial with the path of the electron beam, such aperture in the anode electrode means having an effective diameter several times the diameter of the small aperture in said grid electrode, and means for connecting said anode electrode means to a potential nearer to the potential of the first anode electrode than to the potential of the grid electrode, said anode electrode means acting to strengthen the electric field between the anode electrode means and the grid electrode, while the predetermined electron beam remains attracted toward said first anode electrode.
 3. The cathode ray tube according to claim 2 wherein said anode electrode means overlaps and is at least partially in the same plane with the said grid electrode means, being disposed at least partially in surrounding relation to said grid electrode means.
 4. The cathode ray tube according to claim 2 wherein said anode electrode means is connected to substantially the same potential as said anode electrode.
 5. The cathode ray tube according to claim 2 wherein said grid electrode means comprises an annular shoulder portion mounted on said grid electrode and extending axially of the electron beam toward said anode electrode in surrounding relation to the aperture in said grid electrode.
 6. The cathode ray tube according to claim 2 wherein the aperture in said anode electrode means is larger in diameter than the aperture in said grid electrode means.
 7. The cathode ray tube according to claim 2 wherein said grid electrode means and said anode electrode means each comprise a thin disc provided with the respective apertures therein, each such disc having a thickness on the order of magnitude of the diameter of the aperture in the grid electrode.
 8. The cathode ray tube according to claim 2 wherein the potential of said anode electrode means is between the potential of said first anode electrode and the potential which would correspond to the position of said anode electrode means in a uniform field between the first anode electrode and the grid electrode.
 9. The cathode ray tube according to claim 2 wherein said anode electrode means is closer to the first anode electrode than is said grid electrode means.
 10. the cathode ray tube according to claim 9 wherein the anode electrode means is more closely spaced to said grid electrode means than to said first anode electrode
 11. A cathode ray tube in which the improvement comprises an electron gun structure including: a cathode for emitting an electron beam; a grid electrode adjacent the cathode and having a small aperture through which a predetermined electron beam travels, said grid electrode cooperating to produce an electron beam crossover adapted for focusing upon the screen of said cathode ray tube; a first anode electrode on the remote side of said grid electrode from said cathode for producing an accelerating field for attracting said electron beam, said anode having an aperture for substantially passing said electron beam; means for supplying a relatively high voltage to the first anode electrode which voltage is positive relative to the cathode; annular, , substantially disc-shaped grid electrode means positioned between said first anode electrode and said grid electrode an adjacent said grid electrode, said grid electrode means being provided with an aperture coaxial with the path of said electron beam, such aperture in the grid electrode means having a diameter several times the diameter of the small aperture in said grid electrode, and means connecting said grid electrode means to the same potential as the grid electrode; and annular, disc-shaped anode electrode means positioned between said anode electrode and said grid electrode means in closer spaced relation to said grid electrode means than to said first anode electrode, said anode electrode means being provided with an aperture coaxial with the path of the electron beam, and means for connecting said anode electrode means to substantially the same potential as the anode electrode, said anode electrode means acting to strengthen the electric field between the anode electrode means and the grid electrode, while the electron beam remains attracted toward said first anode electrode.
 12. The cathode ray tube according to claim 11 wherein said grid electrode means comprises an annular shoulder mounted on said grid electrode.
 13. The cathode ray tube according to claim 12 wherein said anode electrode means overlaps and is at least partially in the same plane with the grid electrode means, being disposed at least partially in surrounding relation to said grid electrode means. 